Given a model with the validation below, a produced error will be attached to the hash key :validate_username_unique instead of to :username.

validates_with_method :username, :validate_username_unique

def validate_username_unique

if new_record? || changed?(:username)
  if self.class.by_username( :key => self.username, :limit => 1).blank?
    return true
  else 
    return [false, "Username has already been taken"] 
  end 
else
  return true
end

end

# note: changed?(field) is a method added to the class, that is tested and works

Here is what the error inspection looks like:

<CouchRest::Validation::ValidationErrors:0x23a2e18 @errors={:validate_username_unique=>["Username has already been taken"]}>